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Why isn't adding friends of friends allowed on Facebook?

I’ve been trying to get to know more people from my area on Facebook (since I’m too shy to talk to them in real life) so I sent friend requests for friend of friends but now I found out that that’s not allowed? I got a message saying you can’t add people you don’t know in real life..but is it okay to add friends of friends since they show up my find friends list anyway?